1. Field of the Invention
The present invention relates to quickly and accurately determining the position of an end user using a wireless local area network (WLAN). Alternatively, assuming the end user has recently left the WLAN, the present invention relates to using the WLAN information to more quickly determine the position of the end user using a global positioning system (GPS).
2. Related Art
Location information of a mobile user is increasingly being used in a wide range of situations. For example, such location information can help a person navigate in an unfamiliar area, thereby allowing that person to quickly find a restaurant, shopping center, or some other destination. In addition to being useful, location information can also be critical in emergency situations. For example, location information can be used to reach a person that has dialed 911 from a cellular phone. To ensure that emergency personnel can find that person in need, the FCC mandates that cellular providers report the 911 caller location, known as enhanced 911.
To comply with this mandate, cellular phones can access a system called Global Positioning System (GPS) to provide location information. GPS currently includes a nominal operational constellation of 24 satellites in orbit. The constellation configuration includes six equally spaced orbital planes provided 60 degrees apart and inclined at approximately 55 degrees from the equatorial plane. Of importance, this constellation configuration allows a receiver to establish contact with multiple satellites, e.g. between 4 and 8 satellites, from any point on earth.
Although still controlled by the U.S. Department of Defense, GPS is available to the general public at no charge and provides high location accuracy (e.g. within 100 meters). However, GPS poses two major problems with respect to wireless devices (e.g. cellular phones). First, GPS receivers do not work reliably indoors due to either attenuation (e.g. the signal is too weak to be received) or reflection (e.g. the GPS receiver inadvertently picks up a reflected signal (a phenomenon known as multipath), and therefore computes the wrong position).
Second, even under ideal conditions, a GPS receiver can take a significant period of time to compute its position. Specifically, conventional GPS receivers need to decode GPS satellite data before the first position can be computed. Because the current data rate for a GPS signal is only 50 bit per second, the GPS receiver may require up to 12.5 minutes to collect the GPS satellite data necessary in a “cold start” to determine the first position. A cold start means that both the GPS almanac, i.e. the approximate orbital data parameters for the satellites, and a rough position of the device are not known. Moreover, even in a “warm start” in which the GPS almanac and rough location are known, but not the GPS ephemeris, i.e. detailed data parameters that describe short sections of the satellite orbits, the GPS receiver may still require approximately 40 seconds to determine the position of the device. This time period may cause users various degrees of adverse results ranging from mere inconvenience to life-threatening situations.
Other positioning techniques have been proposed, but have not been acceptable due to their disadvantages. For example, in a positioning technique called triangularization, a time difference of arrival (TDOA) is calculated to locate a wireless caller. However, triangularization requires at least three reference points (base stations) to get a reasonably accurate position, and typically a cellular phone is covered by only one base station. In another positioning technique provided by Snaptrack, a subsidiary of Qualcomm, a GPS receiver can track satellites indoors using one or more cell phone networks. However, this technique works only within cellular phone coverage and requires modifications of current cell phone infrastructure.
Therefore, a need arises for a method of quickly determining an accurate position of a device using an existing infrastructure.